GB2570886A

GB2570886A - Vacuum metallisation process

Info

Publication number: GB2570886A
Application number: GB1801958.8A
Authority: GB
Inventors: William Jarman Robert
Original assignee: Camvac Ltd
Current assignee: Camvac Ltd
Priority date: 2018-02-07
Filing date: 2018-02-07
Publication date: 2019-08-14
Anticipated expiration: 2038-02-07
Also published as: GB201801958D0; GB2570886B

Abstract

A process comprises operating one or more pumps to evacuate gas from a vacuum chamber 12 while simultaneously monitoring the pressure within the chamber and injecting inert gas into the chamber at a flow rate dependent on the pressure to maintain said pressure at a predetermined level. The process is suitable for increasing the speed or efficiency of a vacuum metallisation process by reducing the time for the chamber to achieve its processing pressure. An apparatus for performing the process of the invention comprises one or more pumps P1, P2, at least one sensor PS for monitoring pressure within the chamber, at least one gas injection device G and a controller C communicatively coupled to the sensor and the injection device. The apparatus preferably comprises first and second rollers R1, R2 positioned to direct a web of film 14 through at least part of the vacuum chamber and at least one metallisation processing station F1, F2, F3 arranged to metallise the web of film. The apparatus may be used to form a multi-layered film having gas or vapour barrier properties for the packaging of oxygen or moisture sensitive foodstuffs for the encapsulation of gas or moisture sensitive components.

Description

Vacuum Metallisation Process and Apparatus

Background

Multi-layer films having enhanced barrier properties for gas and/or vapour can be produced by depositing alternate layers of cured polymer and metal or compounds onto a substrate using processes such as vacuum deposition. These films are useful for packaging of oxygen or moisture sensitive foodstuffs, encapsulation of gas or moisture sensitive components, and a variety of other functional applications requiring barrier properties.

It is known to create multi-layer films such as those described above within a vacuum chamber. This is known in the art as vacuum metallisation.

In a vacuum metallisation process the vacuum level, i.e. the pressure within the vacuum chamber, has a bearing on the quantity of evaporated material which reaches the substrate to be coated. As the vacuum improves, and therefore pressure reduces, the mean free path within the chamber increases as there are fewer molecules of gas within the chamber to cause collisions with the material being evaporated.

A vacuum metallisation chamber can contain contaminant such as aluminium oxide left over from an earlier process. Such contaminant can absorb moisture when the chamber is open to atmospheric conditions. During a subsequent vacuum pump-down cycle, where the vacuum within the chamber is increased prior to metallisation, this moisture outgasses, initially as the vacuum pumping starts, but also as heat from the metallisation process is applied. The effect of this is to cause a pressure rise within the chamber. The vacuum level within the chamber will stabilize as the process starts and will then generally improve throughout the process run as the outgassing reduces.

As vacuum improves, the optical density of the coating being deposited on the substrate increases, indicating that more material is being applied, thereby creating a thicker layer.

In a metallisation process where the deposited layer is visible, the line speed of the substrate can be increased to maintain a constant layer thickness and therefore consistent optical density. The line speed can be controlled manually by an operator, or by a closed loop system using an optical density monitoring system to control the machine speed.

This method cannot be applied however where a two stage deposition system is being used to create a transparent layer, such as aluminium oxide. In such a process the optical density of the metal layer is pre-set before the reactive gas is injected into the metal vapour to form the oxide layer being deposited on the substrate. The control system for the gas injection is used to maintain the much lighter optical density of the oxide layer, while the amount of metal being evaporated is held constant, with no adjustment permitted to the heater power level or the wire feed rate during the process run. The thickness of the metal layer can increase if the vacuum level changes during the run, as described above. This effect can be more critical when running an oxide process because the optical density of the metal layer can be much lighter, so any change in thickness can represent a much larger percentage change in optical density. The reactive gas control system would compensate by allowing more gas in to react with the extra metal, to maintain the optical density of the oxide layer. All the material would be in specification for optical density while running, but there would be a different thickness of coating being applied. As this type of coating continues to fade and become lighter after processing and exposure to atmosphere, this can affect the finished appearance of the substrate produced.

In order to reduce the effect of variation in chamber vacuum level it is common practice to extend the vacuum pumping time before starting a metallisation process to achieve a lower chamber pressure and allow more time for outgassing. When the evaporation source is started, and heat and wire are applied, it is again common practice to wait until the vacuum level has recovered to its ultimate pressure before starting the process, in order to minimize the change in vacuum level during the metallisation run.

The present inventor has devised a vacuum metallisation process that can increase the likelihood of a uniform thickness metal layer and/or can increase the speed and/or efficiency of the process by reducing the time it takes for the system to achieve its ultimate pressure relative to known metallisation processes.

Summary

According to a first aspect of the present invention there is provided a vacuum metallisation process comprising the steps of:

operating one or more vacuum pumps to evacuate gas from a vacuum chamber;

while evacuating gas from the vacuum chamber, monitoring the pressure within the vacuum chamber;

while evacuating gas from the vacuum chamber and/or monitoring the pressure within the vacuum chamber, injecting inert gas into the vacuum chamber at a flow rate that is dependent on the pressure within the vacuum chamber to maintain the pressure within the vacuum chamber at a predetermined pressure.

Thus, the method according to the first aspect enables uniformity of the thickness and/or optical density of a metal layer of a metallised film to be improved by dynamically changing the flow rate of inert gas injected into the vacuum chamber under vacuum conditions in order to maintain the pressure at a constant level. As such, the flow rate of the inert gas is modified to account for pressure variations due to outgassing. The method according to the first aspect can also enable the process to be faster as there is no requirement to extend the vacuum pumping time before starting a metallisation process to achieve a lower chamber pressure and allow more time for outgassing, or wait until the vacuum level has recovered to its ultimate pressure before starting the process.

The predetermined pressure can be greater than a typical vacuum pressure at the start of a production run. The typical vacuum pressure at the start of a production run can be referred to as the ultimate pressure. Whereas the ultimate pressure at the start of a production run can be less than or equal to 0.1 Pa and in some cases less than or equal to 0.01 Pa, the predetermined pressure in embodiments of the invention can be greater than 0.01 Pa and in some embodiments greater than 0.1 Pa. Thus the process can be started earlier, saving up to 50 % of pumping time in some cases.

Preferably the predetermined pressure is greater than or equal to 0.2P Pa, more preferably greater than or equal to 0.3 Pa, and more preferably greater than or equal to 0.4 Pa. A greater predetermined pressure can reduce unproductive downtime prior to metallisation.

It is however preferred that the predetermined pressure is less than or equal to 0.5 Pa in order to provide a sufficient vacuum level.

The inert gas can comprise nitrogen and/or argon.

The inert gas can be introduced from an opposite side of the vacuum chamber relative to the one or more pumps. The one or more pumps can be on the same side of the vacuum chamber.

In accordance with a second aspect of the present invention, there is provided apparatus for carrying out the method of the first aspect, the apparatus comprising:

a vacuum chamber;

one or more pumps operable to evacuate gas from vacuum chamber;

at least one pressure sensor arranged to monitor the pressure within the vacuum chamber;

at least one gas injection device arranged to inject a flow of inert gas into the vacuum chamber at a flow rate, the gas injection device including a flow modifier operable to vary the flow rate; and a controller communicatively coupled to the pressure sensor and the gas injection device, the controller being configured to cause the gas injection device to inject inert gas into the vacuum chamber at a flow rate that is dependent on the pressure within the vacuum chamber to maintain the pressure within the vacuum chamber at a substantially constant predetermined pressure.

Optional features of the first aspect can be applied to the second aspect in an analogous manner.

In accordance with a third aspect of the invention there is provided a computer programme product containing code which, when run by the processor of the apparatus of the second aspect, causes the gas injection device to inject inert gas into the vacuum chamber at a flow rate that is dependent on the pressure within the vacuum chamber to maintain the pressure within the vacuum chamber at a predetermined pressure.

Description of the Drawings

By way of example only, certain embodiments of the invention will now be described by reference to the accompanying drawings, in which:

Figure 1 is a diagram of apparatus for carrying out a vacuum metallisation process according to an embodiment of the invention; and

Figure 2 is a flow chart illustrating a vacuum metallisation process according to an embodiment of the invention.

Embodiments of the Invention

By way of a non-limiting overview, embodiments of the present invention relate to a system and method for controlling the vacuum level in a metallisation chamber. The vacuum level is maintained at a predetermined pressure level while the process is running, thereby creating a more consistent metal layer throughout the run. In existing vacuum metallisation processes the vacuum chamber is evacuated until a steady state is reached. The vacuum pumps work to maintain the steady state during metallisation in order to achieve a consistent thickness of an applied metal layer. A vacuum system can be considered to be bipolar in that it is either evacuating and thus heading towards a minimum pressure endpoint, or not evacuating and thus heading towards an atmospheric pressure endpoint. The present inventor realised that by injecting an inert gas into the vacuum chamber during evacuation, the quantity of injected gas can be dynamically varied based on the measured vacuum pressure to set and maintain the vacuum chamber at a pressure which is sufficient for vacuum metallisation but is greater than the minimum vacuum pressure endpoint. As such, a working pressure is reached within the vacuum chamber relatively quickly in comparison to a process which requires ultimate, steady state, vacuum level to be achieved, thereby speeding up the vacuum metallisation process due to a reduction in unproductive waiting time. Moreover, the vacuum level can be maintained throughout a process run in spite of pressure variations due to variable load from the process, thereby providing consistent coating performance.

Figure 1 shows apparatus for carrying out a vacuum metallisation process according to an embodiment of the invention generally at 10.

The apparatus 10 comprises a sealed unit 12 which is hollow to define a vacuum chamber VC.

Within the vacuum chamber VC, there is provided a rotatably mounted first roller R1 and a rotatably mounted second roller R2. A web of film 14 extends from the first roller to the second roller such that it can be wound from the first roller R1 to the second roller R2. As the film 14 passes from the first roller R1 to the second roller R2 it encounters a series of processing stations Fl to F3. The processing stations Fl to F3 can be processing stations as defined in EP925180; for example the first processing stage Fl can comprise condensing and curing a radiation curable precursor material such as acrylate onto the web 14. The subsequent processing stage F2 can comprise metallising the top surface of the cured radiation curable material to produce a vacuum metallised film. The third processing stage F3 can comprise condensing and curing a top layer of radiation curable material onto the metallised surface using a low-energy ion flux directed at the web 14.

In other embodiments, the vacuum metallization process can take any suitable form.

The first and second vacuum pumps Pl, P2 are provided in fluid communication with the vacuum chamber VC. The pumps Pl, P2 are arranged to evacuate gas from the vacuum chamber VC to lower the pressure within the vacuum chamber VC. In this embodiment, the pumps Pl, P2 are located on a first side of the vacuum chamber VC. However, in other embodiments any number of pumps can be provided in any suitable configuration.

The pumps Pl, P2 are capable of lowering the pressure within the vacuum chamber to less than 0.3 Pa. In other embodiments the pumps can be capable of lowering the pressure within the vacuum chamber to less than 0.1 Pa and in some embodiments less than or equal to 0.01 Pa.

A pressure sensor or vacuum gauge PS is provided to measure the pressure within the vacuum chamber VC. Any suitable pressure sensor can be provided; for example, a thermal conductivity gauge such as a Pirani gauge. In this embodiment, the pressure sensor PS is located on a second side of the vacuum chamber VC opposite to the first side of the vacuum chamber VC. However, in other embodiments any number of pressure sensors PS can be provided in any suitable configuration.

The apparatus 10 further comprises a gas injector G arranged to pump inert gas into the vacuum chamber VC while the pumps Pl, P2 are operating to evacuate the vacuum chamber VC. The gas injector G includes a flow modifier such as a mass flow controller operable to vary the flow rate of the inert gas injected into the chamber VC. In this embodiment, the gas injector G is located on the second side of the vacuum chamber VC. However, in other embodiments any number of gas injectors G can be provided in any suitable configuration.

A controller C such as a computing device is communicatively coupled to the pressure sensor PS to receive signals indicative of the pressure within the vacuum chamber VC. The controller C is also communicatively coupled to the gas injector G to provide signals which determine the flow rate of the inert gas injected into the chamber VC.

Referring additionally to Figure 2, in use, at step 20, the pumps Pl, P2 are operated to evacuate gas from the vacuum chamber VC.

At step 22, while the pumps Pl, P2 are evacuating gas from the vacuum chamber VC, the pressure sensor PS generates a pressure signal representative of the pressure within the vacuum chamber VC. In embodiments where a plurality of pressure sensors are provided, the pressure signal can be an average of the readings provided by the plurality of pressure sensors.

At step 24, while the pumps Pl, P2 are evacuating gas from the vacuum chamber VC, the controller C receives the pressure signal and determines whether the pressure within the vacuum chamber is greater, equal to, or less than a predetermined pressure.

Based on the outcome of this determination step, the controller outputs a control signal at step 26 that either instructs the gas injector to decrease, maintain or increase the flow rate of the injected inert gas, in order to account for measured pressure variation within the vacuum chamber VC. This step is also carried out while the pumps Pl, P2 are evacuating gas from the chamber VC.

At step 28, while the pumps Pl, P2 are evacuating gas from the vacuum chamber VC, the gas injector G injects a flow of inert gas into the vacuum chamber VC at a flow rate determined by the control signal from the controller C. Thus, the controller C is arranged to dynamically vary the quantity of inert gas supplied to the vacuum chamber VC to hold the vacuum chamber pressure at a substantially constant level.

The gas injection can be started during the heat up phase of the metallisation process, as soon as the chamber pressure has recovered to below the desired vacuum control set point. This means that the optical density of the metal layer is set with gas injection on, and then maintained throughout the run as the control system adjusts the flow of inert gas to control and maintain the set vacuum level.

Process steps 22 to 28 can be repeated for the duration of the metallisation process.

The controller C can be provided with a user interface (not shown). The required vacuum level set point is readily adjustable through the control system HMI screen. Any suitable user interface can be provided.

Precursors can for example be organic or inorganic and include unsaturated organic materials, silicon-based materials, halogen-based materials, organometallic composites etc, with acrylates such as tripropylene glycol diacrylate or isobornyl acrylate being preferred. Most polymerisable materials described in the art can be used in the process. The vaporised or atomised material may optionally include other radiation curable or non-curable components to provide functionality such as adhesion promotion, dimensional stability, mechanical properties, colour, antibacterial properties, hydrophillia, hydrophobia, electrical conductivity etc.

The thickness of a precursor film or cured polymer coating can be any suitable value. For example, in some embodiments the value may be at least O.OOlpm. In some embodiments, the value is in the range O.OOlpm - 50pm, and preferably O.Olpm to lpm, the preferred thickness largely being decided on the basis of the function of the polymer layer in the intended application, and cost constraints, rather than constraints arising from the process. For example, for barrier packaging applications, the function of the polymer layer is to protect the barrier coating (i.e. the aluminium or aluminium oxide) against physical damage or abrasion. In this case, the lower limit of thickness of the polymer layer may be around 0.02pm, as there is insufficient protection below this. The upper limit may be subjective, as above about 1pm the benefit of mechanical protection will begin to be outweighed by the risk of delamination.

Any web substrate which can be handled by the equipment can be used in the invention. Substrates can include a wide variety of commercially available thermoplastic films (including polyesters such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) or blends or coextrusions thereof), polyamides (including nylon 6 and nylon 6.6), polyolefines (including polypropylene and high and low density polyethylene) and other thermoplastic films known in the art. Non-thermoplastic films, including biodegradable films and films derived from renewable resources, such as polylactic acid or cellulose-based materials including cellulose diacetate, also known as cellulose acetate, may also be used. Thermoset polymer films, such as polyimides may also be used. Fibrous, non-woven or woven substrates (such as paper or textiles) may also be used. The invention is not limited by this list of web substrates.

The process of embodiments of the invention may be a high speed process, meaning that the web substrate is moving at a speed of at least 50 m/min. It is preferred that the web is moving at a speed of at least 5 m/s, and more preferably that that the web is moving at a speed of at least 7 m/s. In some embodiments of the invention, the web may form part of a reel to reel process.

The substrate can optionally be pre-coated or post-coated, vacuum deposited or printed with a wide variety of metals, metallic or non-metallic compounds and other materials, in order to achieve desired properties or effects. For nontransparent barrier applications, for example, substrates such as polyester films coated with a metal such as aluminium are especially preferred. For transparent barrier applications, substrates such as polyester films coated with a transparent metallic or non-metallic oxide, nitride or other compound (e.g. oxide of aluminium or oxide of silicon) are especially preferred. For electrical or electronic applications, the web substrate may be optionally pre-coated with a metal such as copper or another conductive inorganic or organic material, which however may be transparent or non-transparent. However, the invention is not limited to these specified coatings.

For very high barrier applications a plurality of barrier layers separated by polymer layers, can be used, as this extends the diffusion pathway for gas or vapour between the permeable defects in each barrier layer. In this case, since the polymer layer functions as a separating layer between two metal or ceramic layers, and has little or no inherent barrier of its own, it should preferably be as thin as practicable, conducive with the requirements that it should be continuous,

i.e. with no voids or defects, and have good surface smoothness to maximise the barrier of the second or subsequent barrier layer.

Materials manufactured by the invention are suitable for use in multiple different applications including: packaging applications; abrasion-resistant material or intermediate (in which the polymer coating prevents abrasion damage to any underlying functional layers during conversion or use); security or anti-counterfeit applications, including continuously optically variable devices; decorative applications, including continuously optically variable devices; functional industrial applications; and electrical or electronic applications (inclusive of static electricity dissipation).

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications can be made without departing from the scope of the invention as defined in the appended claims. The word comprising can mean including or consisting of and therefore does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A vacuum metallisation process comprising the steps of:

operating one or more vacuum pumps to evacuate gas from a vacuum chamber;

while evacuating gas from the chamber, monitoring the pressure within the vacuum chamber; and while evacuating gas from the chamber, injecting inert gas into the vacuum chamber at a flow rate that is dependent on the pressure within the vacuum chamber to maintain the pressure within the vacuum chamber at a predetermined pressure.

2. A vacuum metallisation process according to claim 1, whereby the predetermined pressure is greater than 0.01 Pa.

3. A vacuum metallisation process according to claim 2, whereby the predetermined pressure is greater than or equal to 0.1 Pa.

4. A vacuum metallisation process according to claim 3, whereby the predetermined pressure is greater than or equal to 0.2 Pa.

5. A vacuum metallisation process according to claim 4, whereby the predetermined pressure is greater than or equal to 0.3 Pa.

6. A vacuum metallisation process according to claim 5, whereby the predetermined pressure is less than or equal to 0.5 Pa.

7. A vacuum metallisation process according to any preceding claim, whereby the inert gas comprises nitrogen and/or argon.

8. A vacuum metallisation process according to any preceding claim, comprising metallising a substrate within the vacuum chamber while performing the step of injecting inert gas into the vacuum chamber.

9.

Vacuum metallisation apparatus comprising: a vacuum chamber;

one or more pumps operable to evacuate gas from vacuum chamber;

at least one gas injection device arranged to inject a flow of inert gas into the vacuum chamber at a flow rate, the gas injection device including a flow modifier operable to vary the flow rate; and a controller communicatively coupled to the pressure sensor and the gas injection device, the controller being configured to cause the gas injection device to inject inert gas into the vacuum chamber at a flow rate that is dependent on the pressure within the vacuum chamber to maintain the pressure within the vacuum chamber at a predetermined pressure.

10. Vacuum metallisation apparatus according to claim 9, further comprising first and second rollers positioned to direct a web of film to be metallised through at least some of the vacuum chamber and at least one metallisation processing station arranged to metallise the web of film within the vacuum chamber.

11. Vacuum metallisation apparatus according to any preceding claim, wherein the predetermined pressure is greater than 0.01 Pa.

12. Vacuum metallisation apparatus according to claim 11, wherein the predetermined pressure is greater than or equal to 0.1 Pa.

13. Vacuum metallisation apparatus according to claim 12, wherein the predetermined pressure is greater than or equal to 0.2 Pa.

14. Vacuum metallisation apparatus according to any preceding claim, wherein the inert gas comprises nitrogen and/or argon.

15. A computer programme product containing code which when run by the processor of the apparatus of claim 9 causes the gas injection device to inject inert gas into the vacuum chamber at a flow rate that is dependent on the pressure within the vacuum chamber to maintain the pressure within the vacuum chamber at a predetermined pressure.